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Tuesday, December 20, 2011

CryoBUG gets a cap tube adjustment

After my last two tests, I really wasn't happy about the temperature I was seeing at the phase separator. Normally when the amount of flow from CT #1, and the amount of R-134a is sufficient,  I should be getting down to at least -20°C (if not colder depending upon how much of the other refrigerants have dissolved into the sub cooled condensate). So what was I seeing? No matter how much refrigerant I had added, I could never get it to go below zero. In fact it tended to always be up in the positive numbers most of the time.

So what to do? Well I decided I needed more flow from CT #1. The only problem is that CT #1 is stuffed inside my Cascade HX, so it's not really replaceable at this point. I decided the next best thing was to add another cap tube in parallel, and insert this from the out side by drilling a hole into the HX, stick the new cap tube through the hole, and braze it in place.

The other problem was how to sub cool this additional cap tube like I had done for the original CT #1. So I needed to get the cap tube and the outer tubing of the HX in intimate contact with each other. Brazing it was the only reliable method I could think of to do this. So although this was going to be a tedious process, that is what I decided to do.

Just me starting to braze the new cap tube in place

Yep this is not easy

CryoBUG HX Stack with 2nd parallel CT #1 brazed to outer tubing

Close-Up View of brazed cap tube

It took me about 3 hours to complete, and although not very pretty, it should definitely do the trick.

Another problem cropped up while I was brazing the cap tube to the outside of the HX. Although I was under constant nitrogen purge, I began to notice a lot of smoke coming out of the loose end of my additional cap tube, and this was also accompanied by a constant drip of oil. This started to concern me that whatever oil still remained coating the inner walls of the HX, might be under going a bad chemical breakdown because of all the heat. So after I finished the brazing, and cooled the stack off with water, I decided I needed to flush out any oil that remained.

The flushing process consisted of evacuating an empty cylinder, sucking it full of Acetone, pressurizing this with nitrogen, and then forcing it through the cap tubes via the discharge line. I captured what came out the suction line into a bucket, which initially looked to be the color of oil (pale yellow). After repeating the flushing process 2 more times, until everything came out as clean as it originally went in. At this point I blasted everything out that I could with high pressure nitrogen and then put the HX stack on a vacuum pump for about an hour to evaporate anything that was left.

So I took the cleaned out stack and re-brazed it back to the condensing unit, and then put it on overnight pump-down. The next day It was sitting at 35 microns. All was once again well in my world.

Opps almost forgot! I also changed the final cap tube in order to reduce it's flow to about half of what it used to be. I figured since I was doubling the flow in CT #1, that I needed to reduce flow elsewhere if I was ever going to maintain the suction pressure where I desired it to be (looking to be around 10-15 psig).

Next on the agenda: Re-charge and test.

2 comments :

Calathea said...

Fascinating stuff indeed. I hope you reach your goal and most of all, I hope someone can use this for PC's.

Michael St. Pierre said...

Hi Calathea,

I'm getting steadily closer to the goal (-150 C). And with my last test, I made it to -140, which isn't really all that far away. As for PC use, I could see this working if it were scaled up to handle the load. Presently I'm doing this with a compressor having only 4.8cc/rev displacement, but obviously my load is very small (perhaps 5-10 watts). Bumping the displacement up proportionally as well as the cap tube flow, could get it into PC territory.

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